1. Field of the Invention
The present invention relates to blends of an ethylene-propylene-diene (EPDM) rubber and a high diene hydrocarbon rubber for use as a replacement for unblended EPDM rubber parts. More particularly, the present invention pertains to blends of EPDM rubber and SBR rubber exhibiting tensile and tear strength comparable to the calculated values for an ideal blend of EPDM and SBR rubber while maintaining the excellent weather, ozone and thermal resistance exhibited by EPDM rubbers.
2. Discussion of the Art
Ethylene-xcex1-olefin-diene rubbers, particularly ethylene-propylene-diene (EPDM) rubbers, are excellent all-purpose rubbers that are useful in a wide variety of applications, including the manufacture of hoses, seals and weather strips. Due in part to the substantial absence of unsaturation in the polymer backbone, EPDM rubbers exhibit superior oxidative and ozone resistance, weather resistance and heat resistance compared to conjugated diene rubbers. In addition, EPDM rubbers compare favorably in price to other elastomers and maintain their properties across a broad range of filler concentrations.
While these polymers provide acceptable performance and exhibit good processability, a tight EPDM supply coupled with an ever increasing market demand for EPDM rubbers drives the search for a low cost EPDM rubber blend that would compare favorably with unblended EPDM rubbers. Toward this end, EPDM has been blended with other elastomers in an effort to develop a rubber with comparable physical properties as unblended EPDM rubber that retains the oxygen, ozone and heat resistance of EPDM while maintaining or reducing the cost of the final composition. These elastomers have included conjugated diene rubbers and polychloroprene. The effectiveness of these compounds is limited by the fact that the weight percent of EPDM that may be added is fairly limited in order to produce a compound with acceptable mechanical properties. In addition, the processing of such compounds is often troublesome and expensive.
Styrene-butadiene rubber (SBR) is a diene rubber that is often considered a candidate for blending with EPDM. It is low in cost and relatively easy to process. Unfortunately, SBR, along with most other diene rubbers, are immiscible in EPDM and exhibit cure incompatibility with EPDM. This cure incompatibility of EPDM and highly unsaturated diene rubbers is demonstrated by the poor performance of the resulting composition in stress-strain tests. In addition, SBR does not tolerate high levels of carbon black and oil incorporation, showing a sharp decrease in its physical properties when even moderately high levels of filler are added. FIG. 1 shows the tensile strengths of EPDM/SBR blends at high carbon black and oil loadings as a function of the concentration of the individual elastomers.
As can be seen, the actual tensile strengths observed for these polymer blends are much lower than the calculated tensile strength of an ideal blend of the two polymers. In fact, such compositions generally perform worse than either pure polymer. This poor performance is due in part to several factors. One cause of this incompatibility is the difference in vulcanization rates. Optimal vulcanization parameters for one of the rubbers will lead to poor vulcanization of the other. This, combined with the preference of various accelerators for one polymer over the other, makes it difficult to achieve satisfactory vulcanization for both polymers. A second factor that contributes to poor vulcanization is the difficulty in achieving uniform dispersion among the two rubbers. Widely different solubility parameters produce poor compatibility between the two rubbers, resulting in difficulty when attempting to mix them to a uniform dispersion. This produces a non-uniform product with irregular properties. Traditional compatibilizers such as terpene resins and surface activated low molecular weight polymers have not been fully effective in mitigating this incompatibility.
A generally accepted industry standard for many rubber parts used in the automotive industry is that the compound must display a minimum tensile strength of 8.0 MPa. This value is now incorporated into the SAE J200 specification for loaded rubber compounds in the automotive industry. Unblended EPDM rubbers can achieve this value over a wide range of filler concentrations. Until now however, high oil and carbon black loaded blends of EPDM rubber blended with a high diene rubber have had difficulty achieving this value.
Therefore, a need remains for a blend of EPDM rubber and a high diene rubber which displays physical and dynamic mechanical properties comparable to unblended EPDM rubber while maintaining ease of processing, moderate cost and heat and oxidative resistance.
The present invention provides a blend of an ethylene-xcex1-olefin-diene rubber and a high diene rubber that exhibits a tensile strength and other properties comparable to the calculated values for an ideal blend of the two polymers while maintaining the thermal and oxidative resistance of an EPDM rubber. The blends exhibit a tensile strength of greater than 8.0 MPa and a compression set after 22 hours at 70xc2x0 C. of less than 20%.
The compounds of the present invention comprise an oil extended high molecular weight EPDM rubber, a diene rubber, and a low molecular weight EPDM rubber with a relatively broad molecular weight distribution, low Mooney viscosity and high ethylene content which serves as a compatibilizer. Also included in the composition are various cure agents and vulcanization accelerators. In addition, the blends may contain conventional additives standard in the industry.
The vulcanized rubbers of the present invention are obtained by processing the composition using conventional equipment and techniques and curing the rubber using conventional vulcanization systems and conditions.